2,461 research outputs found
Martian atmospheric compositional analysis- its biological significance first quarterly progress report, 15 may - 15 aug. 1965
Biological significance of Martian atmospheric compositional analysis, and life detection studies of chemical free energy in surface matte
Quantized Friction across Ionic Liquid Thin Films
Ionic liquids, salts in the liquid state under ambient conditions, are of
great interest as precision lubricants. Ionic liquids form layered structures
at surfaces, yet it is not clear how this nano-structure relates to their
lubrication properties. We measured the friction force between atomically
smooth solid surfaces across ionic liquid films of controlled thickness in
terms of the number of ion layers. Multiple friction-load regimes emerge, each
corresponding to a different number of ion layers in the film. In contrast to
molecular liquids, the friction coefficients differ for each layer due to their
varying composition
Five-Dimensional Eguchi-Hanson Solitons in Einstein-Gauss-Bonnet Gravity
Eguchi-Hanson solitons are odd-dimensional generalizations of the
four-dimensional Eguchi-Hanson metric and are asymptotic to
AdS\mathbb{Z}_p$ when the cosmological constant is either positive or
negative. We find soliton solutions to Lovelock gravity in 5 dimensions that
are generalizations of these objects.Comment: 26 pages, 11 figure
Gravitational dynamics for all tensorial spacetimes carrying predictive, interpretable and quantizable matter
Only a severely restricted class of tensor fields can provide classical
spacetime geometries, namely those that can carry matter field equations that
are predictive, interpretable and quantizable. These three conditions on matter
translate into three corresponding algebraic conditions on the underlying
tensorial geometry, namely to be hyperbolic, time-orientable and
energy-distinguishing. Lorentzian metrics, on which general relativity and the
standard model of particle physics are built, present just the simplest
tensorial spacetime geometry satisfying these conditions. The problem of
finding gravitational dynamics---for the general tensorial spacetime geometries
satisfying the above minimum requirements---is reformulated in this paper as a
system of linear partial differential equations, in the sense that their
solutions yield the actions governing the corresponding spacetime geometry.
Thus the search for modified gravitational dynamics is reduced to a clear
mathematical task.Comment: 47 pages, no figures, minor update
The Lanczos potential for Weyl-candidate tensors exists only in four dimensions
We prove that a Lanczos potential L_abc for the Weyl candidate tensor W_abcd
does not generally exist for dimensions higher than four. The technique is
simply to assume the existence of such a potential in dimension n, and then
check the integrability conditions for the assumed system of differential
equations; if the integrability conditions yield another non-trivial
differential system for L_abc and W_abcd, then this system's integrability
conditions should be checked; and so on. When we find a non-trivial condition
involving only W_abcd and its derivatives, then clearly Weyl candidate tensors
failing to satisfy that condition cannot be written in terms of a Lanczos
potential L_abc.Comment: 11 pages, LaTeX, Heavily revised April 200
An Algorithm to Simplify Tensor Expressions
The problem of simplifying tensor expressions is addressed in two parts. The
first part presents an algorithm designed to put tensor expressions into a
canonical form, taking into account the symmetries with respect to index
permutations and the renaming of dummy indices. The tensor indices are split
into classes and a natural place for them is defined. The canonical form is the
closest configuration to the natural configuration. In the second part, the
Groebner basis method is used to simplify tensor expressions which obey the
linear identities that come from cyclic symmetries (or more general tensor
identities, including non-linear identities). The algorithm is suitable for
implementation in general purpose computer algebra systems. Some timings of an
experimental implementation over the Riemann package are shown.Comment: 15 pages, Latex2e, submitted to Computer Physics Communications:
Thematic Issue on "Computer Algebra in Physics Research
Coincident electron channeling and cathodoluminescence studies of threading dislocations in GaN
We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black–white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated
Neighbours of Einstein's Equations: Connections and Curvatures
Once the action for Einstein's equations is rewritten as a functional of an
SO(3,C) connection and a conformal factor of the metric, it admits a family of
``neighbours'' having the same number of degrees of freedom and a precisely
defined metric tensor. This paper analyzes the relation between the Riemann
tensor of that metric and the curvature tensor of the SO(3) connection. The
relation is in general very complicated. The Einstein case is distinguished by
the fact that two natural SO(3) metrics on the GL(3) fibers coincide. In the
general case the theory is bimetric on the fibers.Comment: 16 pages, LaTe
Conserved Matter Superenergy Currents for Orthogonally Transitive Abelian G2 Isometry Groups
In a previous paper we showed that the electromagnetic superenergy tensor,
the Chevreton tensor, gives rise to a conserved current when there is a
hypersurface orthogonal Killing vector present. In addition, the current is
proportional to the Killing vector. The aim of this paper is to extend this
result to the case when we have a two-parameter Abelian isometry group that
acts orthogonally transitive on non-null surfaces. It is shown that for
four-dimensional Einstein-Maxwell theory with a source-free electromagnetic
field, the corresponding superenergy currents lie in the orbits of the group
and are conserved. A similar result is also shown to hold for the trace of the
Chevreton tensor and for the Bach tensor, and also in Einstein-Klein-Gordon
theory for the superenergy of the scalar field. This links up well with the
fact that the Bel tensor has these properties and the possibility of
constructing conserved mixed currents between the gravitational field and the
matter fields.Comment: 15 page
Lagrangian Variational Framework for Boundary Value Problems
A boundary value problem is commonly associated with constraints imposed on a
system at its boundary. We advance here an alternative point of view treating
the system as interacting "boundary" and "interior" subsystems. This view is
implemented through a Lagrangian framework that allows to account for (i) a
variety of forces including dissipative acting at the boundary; (ii) a
multitude of features of interactions between the boundary and the interior
fields when the boundary fields may differ from the boundary limit of the
interior fields; (iii) detailed pictures of the energy distribution and its
flow; (iv) linear and nonlinear effects. We provide a number of elucidating
examples of the structured boundary and its interactions with the system
interior. We also show that the proposed approach covers the well known
boundary value problems.Comment: 41 pages, 3 figure
- …